Chemical vapor deposition (CVD) reactors are used to form various thin films in a semiconductor integrated circuit. Such CVD reactors can form thin films such as SiO.sub.2, Si.sub.3 N.sub.4, Si or the like with high purity and high quality. In the CVD process of forming a thin film, one or more semiconductor substrates are arranged in a reactor and raw material to be deposited is supplied in the form of gaseous constituents so that gaseous molecules are thermally disassociated and combined in the gas and on a surface of the substrates so as to form a thin film. A plasma-enhanced CVD apparatus utilizes a plasma reaction at a relatively low temperature in order to form a thin film. The plasma CVD apparatus includes a process chamber consisting of a plasma generating chamber which may be separate from or part of a reaction chamber, a gas introduction system, and an exhaust system.
Plasma-enhanced CVD reactors are disclosed in U.S. Pat. No. 4,401,504 and commonly-owned U.S. Pat. No. 5,200,232. Plasma is generated in such an apparatus by a high density microwave discharge through electron-cyclotron resonance (ECR). A substrate table is provided in the reaction chamber, and plasma generated in the plasma formation chamber passes through a plasma extracting orifice so as to form a plasma stream in the reaction chamber. The substrate table may include a radiofrequency (RF) biasing component to apply an RF bias to the substrate and a cooling mechanism in order to prevent a rise in temperature of the substrate due to the plasma action.
A plasma apparatus using high density ECR for various processes such as deposition, etching and sputtering to manufacture semiconductor components is disclosed in U.S. Pat. No. 4,902,934. Such a plasma apparatus includes an electrostatic chuck (ESC) for holding a substrate (such as a silicon wafer) in good thermal contact and in a vertical orientation. The chuck can also be provided with cooling and heating capabilities. In general, such reaction chambers can be operated under vacuum conditions, and the plasma generation chamber can be enclosed by walls which are water-cooled. Other types of reactors in which deposition can be carried out include parallel plate reactors and high density transformer coupled plasma (TCP.TM.), also called inductively coupled plasma (ICP), reactors of the type disclosed in commonly owned U.S. Pat. Nos. 4,340,462 and 4,948,458.
In semiconductor processing, devices are being built with smaller wiring pitches and larger interconnect resistances. In order to reduce delays in critical speed paths, it has been proposed to embed low dielectric constant material between adjacent metal lines or lower the dielectric constant of the intermetal dielectric material by adding fluorine thereto. A paper presented at the Feb. 21-22, 1995 DUMIC Conference by L. Qian et al., entitled "High Density Plasma Deposition and Deep Submicron Gap Fill with Low Dielectric Constant SiOF Films" describes deposition of up to 10 atomic % fluorine-containing moisture resistant SiOF films on a silicon sample at room temperature using high density plasma. This paper states that fluorine in the film can be reduced by adding hydrogen to the SiF.sub.4 +O.sub.2 +Ar deposition gas chemistry, the film had a dielectric constant of 3.7, and the refractive index was lowest for deposition conditions where the SiF.sub.4 :SiF.sub.4 +O.sub.2 gas flow ratio was 0.29.
Another paper presented at the DUMIC Conference is by D. Schuchmann et al., entitled "Comparison of PECVD F-TEOS Films and High Density Plasma SiOF Films." This paper mentions that fluorinated TEOS films have been used for gap filling and compares such films to films deposited by inductively coupled high density plasma (HDP) using SiF.sub.4 +O.sub.2 +Ar. The HDP films were found to have better moisture and thermal stability than the F-TEOS films.
Other papers presented at the DUMIC Conference include "Preparation of SiOF Films with Low Dielectric Constant by ECR Plasma CVD" by T. Fukada et al., "An Evaluation of Fluorine Doped PETEOS on Gap Fill Ability and Film Characterization" by K. Hewes et al., "Dual Frequency Plasma CVD Fluorosilicate Glass Water Absorption and Stability" by M. Shapiro et al., and "Water-absorption mechanisms of F-doped PECVD SiO.sub.2 with Low-Dielectric Constant" by H. Miyajima et al. Of these, Fukada discloses that SiOF films deposited by RF biased ECR plasma are superior to SOG and TEOS-O.sub.3 films, the SiOF films providing excellent planarization and sub half micron gap filling without voids. Moreover, according to Fukada, the dielectric constant of SiOF films can be reduced from 4.0 to 3.2 by increasing the SiF.sub.4 /(SiF.sub.4 +SiH.sub.4) gas flow ratio in an RF-biased ECR plasma CVD process using SiF.sub.4, SiH.sub.4, Ar and O.sub.2 gas reactants (O.sub.2 /(SiF.sub.4 +SiH.sub.4)=1.6) and a substrate held on a water cooled electrostatic chuck. Fukada also discloses that the deposition rate was 5500 A/min under the condition of SiF.sub.4, Ar and O.sub.2 without SiH.sub.4. Hewes discloses CVD of fluorosilicate glass films from TEOS, O.sub.2 and C.sub.2 F.sub.6 introduced into a reaction chamber by a showerhead gas mixer. Shapiro discloses that ULSI device speed can be increased by reducing capacitance of the interlevel insulator such as by adding fluorine to SiO.sub.x films such as by using a fluorocarbon gas as the fluorine precursor in TEOS CVD. Miyajima discloses that water absorption of F-doped SiO.sub.2 films containing more than 4% F is a serious problem because it causes degradation of device reliability and film adhesion properties. Miyajima also discloses that the resistance to water absorption is lower for films deposited by parallel plate plasma CVD compared to high density helicon-wave plasma using TEOS, O.sub.2 and CF.sub.4 as deposition gases.
During processing of semiconductor wafers it is conventional to carry out periodic in-situ cleaning of plasma CVD reactors. U.S. Pat. No. 5,129,958 discloses a method for cleaning a CVD deposition chamber in a semiconductor wafer processing apparatus wherein fluorine residues in the chamber, left from a prior fluorine plasma cleaning step, are contacted with one or more reducing gases such as silane (SiH.sub.4), ammonia, hydrogen, phosphine (PH.sub.3), diborine (B.sub.2 H.sub.6), and arsine (AsH.sub.3). The fluorine residues are contacted by the reducing gas or gases for ten seconds to five minutes or longer and the reducing gas or gases are introduced into the chamber at a rate of 100 sccm to 500 sccm or higher while maintaining the temperature in the chamber at 250-500.degree. C. In an example, the chamber was maintained at 1-20 Torr during the reaction between the reducing gas or gases and the fluorine residues or the pressure can range from 10.sup.-3 Torr to 100 Torr during the reaction. Another technique for cleaning and conditioning interior surfaces of plasma CVD reactors is disclosed in commonly owned U.S. Pat. No. 5,647,953, the subject matter of which is hereby incorporated by reference.
Other techniques for cleaning plasma reaction chambers are disclosed in commonly owned U.S. Pat. No. 5,356,478; in U.S. Pat. Nos. 4,657,616; 4,786,352; 4,816,113; 4,842,683, 4,857,139; 5,006,192; 5,129,958; 5,158,644 and 5,207,836 and Japanese Laid-Open Patent Publication Nos. 57-201016; 61-250185, 62-214175, 63-267430 and 3-62520. For instance, in order to remove SiO.sub.x deposits, a fluorine-containing gas energized into a plasma has been used to clean interior surfaces of the chamber. Fluorine residues remaining after the reactor cleaning can be removed by passing a reducing gas such as hydrogen (H.sub.2), silane (SiH.sub.4), ammonia (NH.sub.4), phosphine (PH.sub.3), biborine (B.sub.2 H.sub.6) or arsine (AsH.sub.3) through the reactor.
There is a need in the semiconductor processing industry for a plasma CVD precoat process which minimizes particle contamination and/or provides process repeatability during deposition of SiOF films on semiconductor substrates.